Most microwave filters built using microstrip transmission lines are not effective at suppressing second, third and fourth harmonic signals. Traditionally, the way to solve this problem is to add a lowpass filter at the two ends of a bandpass filter. Physically, this makes the filter structure undesirably bigger. Electrically, using lowpass filters increases signal loss, and the suppression of the harmonics for the most part is not sufficiently effective.
Conventional microwave filters that are capable of suppressing such harmonics have been proposed. U.S. Pat. No. 7,145,418 to Akale et al., the entire content of which is incorporated herein by reference, describes an edge coupled bandpass filter capable of suppressing harmonics. However, some filter applications can require use of different pass frequencies. One way to meet this need is to use a separate filter for each pass frequency. However, the use of multiple filters can be inefficient and expensive. Therefore, a tunable microwave bandpass filter is desirable.